Natural polymers e.g. starch and its derivatives, dextrin, alginates, lecithin, casein, gelatin, soybean protein and some synthetic polymers like methyl-, carboxymethyl- or hydroxyethyl-cellulose, polyvinyl alcohol, polyacrylic acid and other hydrophilic polymers or their combinations are widely used for manufacturing of cellulose composite materials, like sized and coated paper, paper-and cardboard etc. Description of manufacturing processes along with resulting products can be found in many references for example in Encyclopedia of Industrial Chemistry. Vol.A18, 1996; in U.S. Pat. Nos. 4,133,784, 5,057,570, 5,328,567 and 5,527,852. It is well known to add hydrophilic polymers to the pulp for sizing of cellulose material. The hydrophilic polymers are also used for coating the surface of cellulose materials. Usually the coating is applied on cellulose material in a form of aqueous gel and then dried. The coated cellulose materials are in fact composites and since the material of coating is biodegradable such composites are environmentally friendly. The disadvantage of such composites is associated with the fact that they are hydrophilic and not stable against humidity, water and aqueous solutions.
U.S. Pat. Nos. 4,863,655 and 5,362,776 disclose preparation of cellulose composite materials for packaging which comprise cellulose fibers and hydrophilic binders like starch, gelatin, polyvinyl alcohol, polyethylene glycol and polyethylene oxide. Despite these materials are biodegradable they are not sufficient waterproof.
Cellulose composite materials having various hydrophobic protected coating layers have been proposed. Such protected coatings contain various compounds, e.g. polyolefin and additives (U.S. Pat. No. 5,296,307), copolymers of olefins and unsaturated carboxylic acids and pigments (U.S. Pat. No. 3,970,629); a mixture of polyvinyl chloride and ethylene-acrylic copolymer (U.S. Pat. No. 4,365,029). In U.S. Pat. Nos. 3,985,937, 4,117,199, 4,395,499, 4,503,185, 4,599,378, 4,657,821, 5,763,100 and 5,744,547 are disclosed hydrophobic coatings for protection of cellulose substrates containing aqueous latex of synthetic rubbers, polyvinyl esters, polyacrylates, various copolymers, paraffin wax, organically acids, fillers and some other additives. The coatings were applied on cellulose substrate in a form of aqueous latex and dried then. These cellulose composite materials are waterproof, however their biodegradability is not sufficient and therefore they cause pollution of the environment.
There are known also papers treated by silicon organic substances (U.S. Pat. Nos. 3,856,558 and 4,349,610). These coated papers are sufficiently waterproof however they are bio-stable and thus polluted the environment.
U.S. Pat. No. 5,053,268 discloses composite paper containing sheets of synthetic film and paper bonded with urethane adhesive. This composite was water-resistance, but not biodegradable. Some hydrophobic biodegradable polymers and co-polymers like poly-.beta.-hydroxyalkanoates (PHA) have been used as coating layers for manufacturing composite cellulose materials (Witt U. et al., Biologisch abbaubare Polymere, Braunschweig, 1997, p.151). These polymers or co-polymers contain ester links that can be destroyed by microorganisms of the environment. PHA in melted state was applied on the cellulose substrate at temperatures above 150-160 degrees C. Upon cooling a solid water-resistant PHA layer was formed on the cellulose substrate. Nowadays such polymers and co-polymers of PHA type are produced in small amounts from rather rare raw materials. The manufacturing technology is complicate and thus the cost of such materials is high (US $ 20-40 per kg). It should be pointed out that the biodegradation process of PHA is relatively slow. For example PHA destruction in soil takes more than 8 months. Furthermore the application of PHA on cellulose substrate at the above mentioned temperatures is associated with its thermal destruction.
Coating of cellulose substrate by aqueous dispersions (latexes) of PHA is known in the art (Lauzier C. A., et al.--Tappi Journal, 1993, vol.76, No 5, p.71-77; U.S. Pat. No. 5,350,627). The cellulose substrate was impregnated by PHA latex, then dried under room temperature and pressed for few minutes at 100-140 degrees C. that is below the melting point of PHA. Due to the low pressing temperature and short period of time it is possible to avoid thermal destruction of cellulose. However since the particles of PHA do not melt it is not possible to obtain a monolithic and homogeneous coating. As a result the paper composite has loose, porous coating layers and thus decreased waterproofing and reduced water resistance. Another disadvantage of the above mentioned method is associated with the necessity to treat the paper by dilute aqueous dispersion of PHA (5-20% or 50-200 g/l). This causes considerable swelling, warping and possible mechanical damage of initial hydrophilic and porous cellulose substrate during the further steps of the production process. Moreover, PHA is very expensive raw material.